[Antennas] DCTL Antenna Question

[Robert Lay]

Dear Durwydd,

To the best of my knowledge, the folded dipole made from 450 ohm
transmission line is not going to have a feedpoint impedance anything like
450 ohms. At resonance it may be much more and it may be much less.

It has to do with the spacing and wire size, and the formulas that are
around are not going to account very well for the insulation material used
in "window line". I have built folded dipoles of open wire (home built)
transmission line and I've built them from window line. Generally speaking,
you get a feed point impedance that you have to deal with and you just deal
with it in the usual way. It would be foolhardy to presume that the
impedance would in any way correlate with the line's characteristic
impedance when used as a transmission line.

What I do is build the antenna, attach the feedline, run it to the shack,
and then I measure the impedance at the frequencies of interest with my
General Radio RF Bridge. If my tuner cannot handle the impedance that I see,
then I put corrective, lumped inductors or capacitors (preferably inductors)
into position between the xmission line and the tuner. Using General Radio
Banana pin adapters I can plug in and unplug corrective elements at the drop
of a hat band changes, etc.  Works well for me.

Bob Lay (W9DMK), Dahlgren, VA
w9dmk@crosslink.net
http://www.qsl.net/w9dmk